1. Field of the Invention
This invention is directed to a lock-up clutch slip control device to carry out control so that the difference in the rotational speed of input and output axes of a fluid coupling of an automatic transmission converges on a target slip amount.
2. Description of Related Art
Generally, with automatic transmissions that include a lock-up mechanism, a lock-up clutch is in a totally locked state at high speeds in order to reduce transmission loss in a fluid coupling and thus improve fuel consumption. For this reason, there is a direct connection between input and output axes of the fluid coupling. However, at low speeds there is a need for torque amplification action in the fluid coupling. Moreover, at low speeds the lock-up clutch is in a released state to prevent vehicle vibration due to torque fluctuation in the internal combustion engine. This can lead to increased fuel consumption. Thus, in conventional devices, during low speeds the slip amount (i.e., the difference in the rotational speed of the engine and the turbine), and a solenoid duty valve is employed to control the pressure of oil supplied to the lock-up clutch to reach a target slip amount (target value), thus improving fuel consumption.
Japanese Non-Examined Patent Publication No. Hei. 1-206159 discloses a system where the target slip amount is set according to mutually changing multiple patterns based on the driving state of the internal combustion engine to prevent a reduction in drivability.
Japanese Non-Examined Patent Publication No. Hei. 3-14965 discloses a system where based on the difference detected between the input torque and input and output rotational speed, occurrence of an acceleration malfunction is determined after a set time and, if an acceleration malfunction occurs, the target differential oil pressure to the locking clutch is compensated to provide stable slip control.
The two systems described above assume that a pre-set target slip amount is always correct, and slip control is carried out based on that value. However, due to (for example) initial variations in the target slip amount or changes in vehicle characteristics together with changes over time of automatic transmission characteristics, the target slip amount will not always be correct and there could be a reduction in drivability based on acceleration malfunctions, etc. According to the latter system, occurrence of an acceleration malfunction is determined based on whether the target slip amount is reached after a set time. Thus, an increase in throttle aperture could occur for that time and lead to a reduction in drivability and fuel consumption.